1. Field of the Invention
This invention relates to a method of fabricating a multilayer ceramic substrate by stacking unfired ceramic greensheets on either one or both of sides of a previously fired ceramic substrate.
2. Description of the Related Art
Multilayer ceramic substrates have conventionally been fabricated by stacking greensheets. In the greensheet stacking process, after via holes have been formed in a plurality of ceramic greensheets, the via holes of each sheet are filled with conductor paste so that via conductors are formed, and a wiring pattern is printed on each ceramic greensheet using conductor paste. Thereafter, these ceramic greensheets are made by the greensheet stacking process and thermo-compression bonding into a raw substrate. Subsequently, the raw substrate is fired to be fabricated into a multilayer ceramic substrate.
However, about 15 to 30% firing shrinkage occurs in a process of firing the raw substrate. This renders control of dimensional accuracy in the substrate difficult. Moreover, since a shrinkage stress of both sides of the substrate become non-uniform in a multilayer ceramic substrate which has irregularity such as cavity, warp tends to occur in the fired substrate. In particular, warp becomes larger in a bottom of the cavity.
Further, firing temperatures of both types of ceramic greensheets need to be equalized when a composite multilayer ceramic substrate is fabricated by stacking insulating ceramic greensheets and other ceramic greensheets made from material differing in a dielectric substance and magnetic substance. Furthermore, since delamination needs to be prevented by reducing difference in the behavior in the firing shrinkage, the freedom in material selection and accordingly the freedom in the design of the substrate are very limited.
A firing process has recently been proposed reducing the firing shrinkage of the substrate thereby improving the dimensional accuracy of the substrate, as shown in JP-A-2001-267743. In this firing process, an unfired ceramic greensheet on which a wiring pattern is previously printed is stacked on a previously fired alumina substrate to be further processed by thermo-compression bonding. A stack of the greensheet and the substrate is then fired to be fabricated into a multilayer ceramic substrate. In this method, the firing shrinkage of each ceramic greensheet is restrained by the previously fired alumina substrate, whereby the firing shrinkage of the entire substrate is reduced.
However, the results of an experiment conducted by the inventor reveals that a shrinking force of the greensheet is so large that the firing shrinkage thereof cannot sufficiently be restrained even when only the previously fired alumina substrate is applied to one side of the ceramic greensheet. As a result, peeling occurs between a fired layer of ceramic greensheet and previously fired alumina substrate, a crack occurs on the fired layer of ceramic greensheet, and warp occurs in the substrate, whereupon a yield of the products is reduced.
Further, as a firing process reducing the firing shrinkage of the substrate thereby to improve the dimensional accuracy thereof, processes for firing under pressure have been developed as shown in WO91/10630 and JP-A-9-92983. In these firing processes, a restricting alumina greensheet which is not fired at a firing temperature (800 to 1000° C.) of a low-temperature firable ceramic is stacked on both sides of a low-temperature firable ceramic substrate (raw substrate) before the low-temperature firable ceramic is fired. In this state, the raw substrate is fired at a temperature ranging from 800 to 1000° C. under pressure. Subsequently, remainders of the restricting alumina greensheets are eliminated from the sides of the fired substrate by a blasting process etc., whereby a low-temperature firable ceramic substrate is fabricated.
However, when a low-temperature firable ceramic substrate with a cavity is fired by the above-mentioned firing under pressure, pressure applied via the restricting alumina greensheet to a cavity area acts concentrically on a peripheral edge of the cavity, and no pressure is applied to a bottom of the cavity. As a result, the cavity bottom is warped into a convexity and accordingly, the dimensional accuracy of the cavity cannot be ensured.